1. Field of the Invention
The present invention generally relates to a fabrication method of a sensor device, and more particularly relates to an integrated capacitive pressure sensor array manufactured by micromachining technology and CMOS process for fingerprint sensing.
2. Description of the Prior Art
There are many known techniques of identifying an individual through the identification of the individual""s fingerprint. The use of an ink pad and the direct transfer of ink by the thumb or finger from the ink pad to a recording card is the standard way of making this identification. Then, an optical scanner scans the recording card to get an image, which is then compared to fingerprint images in the computer database. However, the most serious drawback of the above-mentioned method is that the fingerprint identification cannot be processed in real-time, and thus cannot satisfy the requirement of real-time authentication, such as network authentication, e-business, portable electronics products, personal ID card, security system, and the like.
The method for reading a fingerprint in real-time has become the important technology in the biometrics market. Conventionally, an optical fingerprint sensor may be used to read a fingerprint in real-time, which can be referred to in U.S. Pat. Nos. 4,053,228 and 4,340,330, and the development is quite mature and accurate. However, the optical fingerprint sensor has a drawback because it is large in size.
Consequently, silicon fingerprint sensors, which overcome the drawbacks of the optical sensor and are formed by silicon semiconductor technology, have been developed. According to the consideration of silicon integrated circuit (IC) processes, the capacitive fingerprint sensor has become the most direct and simple product, which is referred to in U.S. Pat. Nos. 4,290,052 and 4,353,056. However, the problem with the capacitive fingerprint sensor is that it does not effectively overcome the interference problem caused by moisture on the finger and ESD damage to the sensor circuits.
The latest method utilizes capacitive pressure sensor array fabricated by micromachining technology, as the detecting method of the fingerprint. The related material is referred to in appendix 1 xe2x80x9cA High Density Capacitive Pressure Sensor Array For Fingerprint Sensor Applicationxe2x80x9d disclosed by Rey et al.; appendix 2 xe2x80x9cA Very High Density Bulk Micromachined Capacitive Tactile Imagerxe2x80x9d disclosed by De Souza et al.; and appendix 3 xe2x80x9cMEMS Fingerprint Sensor With Arrayed Cavity Structuresxe2x80x9d disclosed by Sato et al., which utilize the pressure pressed from the ridge of the fingerprint for the sensing principle to effectively overcome the above mentioned moisture problem of the capacitive fingerprint sensor. However, the methods disclosed by Rey, De Souza, et al. can not be effectively integrated into the integrated circuit process. Hence, it is not feasible for actual use. The post-IC method disclosed by Sato et al. utilizes an electroplated gold material and sacrificial layer technology to form the pressure sensor structure. However, it increases the process complexity, reduces the yield, and increases the cost. Furthermore, the gold material is not compatible with silicon integrated circuit processing and causes pollution problems.
An object of the present invention is to provide a fabrication method of a pressure type fingerprint sensor, which comprises a plurality of capacitive pressure sensors arranged in a 2-D array for reading the fingerprint, to overcome interference problems caused by the moisture of a finger and to overcome the ESD damage to the sensor circuit. The fabrication method of the present invention is completely compatible with commercial integrated circuit processes, especially the CMOS process and material. Additionally, the present invention can improve the yield and reduce the cost.
Another object of the present invention is to form a pressure type fingerprint sensor, which applies the charge sharing principle to detect the capacitance variation of each capacitive pressure sensor.
In order to achieve these and other objects, the present invention discloses a fabrication method of a pressure type fingerprint sensor comprising a pressure sensor array and a set of processing circuitry. Wherein, the pressure sensor array comprises a plurality of pressure sensors arranged in a 2-D array. Each of the pressure sensors further comprises a plate sensor capacitor comprising a floating electrode, an air gap, and a fixed electrode; a reference capacitor connected to the fixed electrode; and a signal reading unit arranged beside the sensor capacitor for reading the sensor capacitance and connecting to the processing circuitry. Wherein, the floating electrode is used as a contacting surface on which the finger is positioned. The pressure pressed from the ridge of the fingerprint changes the spacing of the air gap so as to change the sensor capacitance. The sensor capacitor further includes a protrusion arranged at a central portion of the contacting surface as a stress concentration point to enhance the displacement of the floating electrode when contacted by the finger ridge. There is further a protection layer formed on the most outer surface for wearing and chemical resistance purposes.
Other aspects, features, and advantages of the present invention will become apparent, as the invention becomes better understood by reading the following description in conjunction with the accompanying drawings.